PURPOSE: To prevent the adherence of water organisms to the surface of an underwater structure by covering the surface of underwater structure with a porous cloth, net, or sheet having a given thickness and capable of easily inducing water replacement.

CONSTITUTION: The surface of an underwater structure, e.g., buoys of floating lamp and raft, fishing net, etc., is covered with a cloth, a net, or a porous sheet having a thickness of 1mm or more, which is free of dissolution, denaturation, and deterioration even when being immersed in water for long periods more than 12 months and capable of water replacement through its voids. The cloth used includes plain weaves, twill fabrics, cotton duck, etc., using various fibers, the net used includes doubly lapped ones, and the porous sheet used includes porous sheets such as urethane foam, etc. The adherence of water organisms to the surface of underwater structure can thus be prevented.